The combined approach to dentofacial deformity using orthodontics and surgery can produce dramatic changes in morphology. Increasing numbers of patients are being treated in this way throughout the United States. Successful post-surgical physiologic adaptation in most patients permits unimpeded speech. However, speech patterns alter somewhat for most patients and some patients fail to make adequate compensatory adaptations for normal speech. In the absence of quantifiable data on form-function interrelations, individual response will continue to be unpredictable. Our preliminary studies on the effects of oral cavity size reduction on formant patterns suggest qualitative and quantitative associations between the nature of adaptive response and the size and geometric relationships between intermaxillary space and tongue. Rapid and dramatic response follows even small changes in oral volume. This study will relate the morphologic changes which result from a variety of surgical approaches to the patients' speech response. Hypotheses concerning structural change, stability and adaptation will be tested and data obtained which could render existing techniques of speech analysis as useful adjuncts to routine clinical diagnosis and treatment planning for orthognathic surgery.